This invention relates to an electronic circuit device comprising an epoxy-modified aromatic vinyl-conjugated diene block copolymer. The copolymer can be included in a resin composition that can be used as, for example, an adhesive, a protective covercoat, or an encapsulant.
A major trend in the electronics industry is to make products smaller, lighter and faster while maintaining or improving their functionality. One of the key technologies that is enabling the creation of more and more compact products is electronic packaging and assembly technology. Electronic packaging and assembly constitutes the materials and processes required to interconnect a semiconductor chip (IC chip) to other electronic or electrical components. Besides the semiconductor chip, various materials can be incorporated into an electronic package, such as flexible circuitry (metal circuitry on polyimide or other polymer films), metal stiffeners, electrically conductive layers, electrically insulating layers, and heat sinks. Often adhesives are used to adhere these various substrates together and to adhere together multiple layers of circuitry on substrates to form multilayered electronic structures having increased wiring density such as may be required for mulitchip modules. Covercoats and encapsulants are needed to protect the circuitry from harsh environments that may be seen in the products"" application. Electrical connection of these packages to printed circuit boards or other electronic components requires that the package undergoes solder reflow, which can expose the package to temperatures of 220xc2x0 C. for tens of seconds to minutes. This process has proved to be quite demanding on adhesives and cover coats, causing product failures such as voiding and/or delaminating from the various substrates. xe2x80x9cPopcorningxe2x80x9d is the generic term coined for these failures. The presence of moisture in the package followed by rapid heating to solder reflow temperatures promotes xe2x80x9cpopcorningxe2x80x9d. Moisture is sorbed by many organic substrates such as polyimide films and organic adhesives, and it can sorb to metals and inorganic components.
Joint Industry Standard, xe2x80x9cMoisture/Reflow Sensitivity Classification for Plastic Integrated Circuit Surface Mount Devicesxe2x80x9d, October 1996, J-STD-020, developed by the Electronic Industries Association""s Joint Electron Device Engineering Council (JEDEC) and the Institute of Interconnecting and Packaging Electronic Circuits, is the electronics industry standard for testing the solder resistance of packages after exposure to humidity. This standard grades materials by levels and JEDEC level 1 (85xc2x0 C./85% R.H. for 168 hours followed by solder reflow at a peak temperature of 220xc2x0 C. for 10 to 40 seconds) is the most demanding test. Packages that pass JEDEC level 1 have the highest degree of solder resistance, and need not be packaged so as to protect them from ambient moisture. Failure at level 1, but passage at less-demanding levels, requires protective packaging.
The current state of the art in adhesive development allows the achievement of JEDEC level 3 (30xc2x0 C./60% R.H. for 192 hours followed by solder reflow at a peak temperature of 220xc2x0 C.) and a very limited number of adhesives claim successful performance of JEDEC level 2 (85xc2x0 C./60% R.H. for 168 hours followed by solder reflow at a peak temperature of 220xc2x0 C.). Specific polyimides can pass JEDEC level 1, but these materials require extremely high lamination temperatures of approximately 350xc2x0 C. Such lamination temperatures prohibit polyimide use in packages that contain materials that will decompose or be altered at these high temperatures, such as organic covercoats, solder masks, solder, etc. In cases where all materials present in the package at the time of lamination can withstand the high laminating temperatures, polyimides can be employed. However, these high temperatures make for a difficult and expensive process.
Epoxidized styrene-diene block copolymers, such as epoxidized styrene-butadiene or epoxidized styrene-isoprene block copolymers, have been described in U.S. Pat. No. 5,478,885. In some applications, the epoxidized block copolymers have been used as rubber toughening agents for commonly-used epoxy resins. Typically, the toughening agent constitutes a small percentage of the total composition.
Cured compositions comprising epoxy resins and epoxy-modified aromatic vinyl-conjugated diene block copolymers have been described in EP658603. The compositions can contain from 5 to 95 parts by weight of an epoxy resin, preferably from 20 to 80 parts by weight. When the composition contains less than 5 parts by weight of either component, a loss of mechanical properties is reported. Use of these materials in electronic packaging applications is not suggested.
Crosslinking epoxidized styrenexe2x80x94diene block copolymers through the backbone epoxy groups has been shown to produce adhesive compositions for use in, for example, pressure-sensitive adhesive tapes, labels, sealants, and coatings as disclosed in U.S. Pat. Nos. 5,229,464 and WO 97/30101. The adhesives can be formulated to include reactive diluents, including epoxy resins, in the amount of from 1 to 50% by weight of the total composition. Use of these materials in electronic packaging applications is not suggested.
Compositions comprising cured epoxidized styrenexe2x80x94diene block copolymers useful as multilayered molding materials have been described in WO 98/22531. The compositions can be cured by a wide variety of known epoxy curing agents. Compositions further comprising added polyfunctional co-reactants, in amounts ranging from 0.01 to 25 parts by weight, are also described. Epoxy resins are not disclosed as co-reactants. Use of these materials in electronic packaging applications is not suggested.
Adhesives useful in electronic packaging comprising liquid epoxy resins and an added functionalized resin have been described in EP 387066 (see also U.S. Pat. No. 5,843,251). In order to extend the shelf life of the adhesives, a micro-capsule type curing agent is required in the formulation. Functionalized styrenexe2x80x94diene block copolymers are among the functionalized resins named and not exemplified, wherein the functional group can be an epoxy group. The added functionalized resin can be present in the amount of from 20 to 80% by weight. The adhesives can be processed into adhesive tapes that can be used to electrically connect circuits on chips and wiring substrates. Performance of the adhesives under conditions of JEDEC Level 1 is not described.
Amine crosslinkable hot-melt adhesives useful in electronics applications, comprising a polyolefin having an epoxy group in its molecule, have been described, for example, in WO 96/33248, wherein an aromatic amine curative is required and the composition optionally comprises an epoxy resin. Polyolefins mentioned are copolymers of ethylene and a glycidyl (meth)acrylate group-containing monomer, so that the epoxy groups are pendant from the polymer backbone. Epoxy-modified aromatic vinyl-conjugated diene block copolymers are not suggested. When present, the amount of added epoxy resin is from about 5 to about 200 parts by weight per 100 parts of the polyolefin copolymer. The adhesives were used for electronic systems, but their use under conditions of JEDEC Level 1 is not suggested.
Briefly, this invention provides an electronic circuit device comprising a resin composition including curable epoxy-modified aromatic vinyl-conjugated diene block copolymer present in the range of 90 to 100 weight percent of the weight of the epoxy bearing material exclusive of curative, optionally up to 10 weight percent of an epoxy resin based on the weight of epoxy-bearing material, and an effective amount of an epoxy curative.
Upon curing with one or both of heat (i.e., temperature up to about 250xc2x0 C.) and UV radiation, the copolymer resin composition and adjacent substrate(s) exhibit superior solder or heat resistance and moisture insensitivity. In particular, the cured resin composition is stable (i.e., does not void or delaminate from the substrate(s)) after exposure to conditions of 85xc2x0 C. and 85% relative humidity (R.H.) for 168 hours followed by rapid heating to a temperature of 220xc2x0 C. for 10 to 40 seconds.
In another aspect, this invention provides a method of using a curable resin composition in an article comprising the steps of:
providing a curable resin composition on at least one surface of a substrate, said resin composition including in the range of 90 to 100 weight percent of an epoxy-modified aromatic vinyl-conjugated diene block copolymer, optionally up to 10 weight percent of an epoxy resin, the weight of both components based on the weight of epoxy-bearing materials, and an effective amount of an epoxy curative, and
incorporating said resin composition as an adhesive, covercoat, or encapsulant in an article, which preferably can be an electronic circuit device.
The resin composition can be a layer or it can be in bulk form.
Upon curing the copolymer with one or both of heat and UV radiation the cured copolymer resin composition exhibits superior solder resistance and moisture insensitivity, as defined above.
The cured copolymer resin composition can be an adhesive, an encapsulant, or it can be a protective covercoat for an electronic circuit device. Examples of the use of the adhesive in electronic circuit devices include: adherence of a layer of flexible circuitry to another layer of flexible circuitry or to a metal stiffener or to a semiconductor chip; adherence of copper or other metallic foil to a polymer substrate; and adherence of an electronic component such as a semiconductor chip to a circuit on a substrate. The adhesive may be insulating or it can be made electroconductive by the addition of electroconductive particles.
In this application:
xe2x80x9celectronic circuitxe2x80x9d means the path of an electric current or electrons and can include such elements as electric conductors, e.g., metallic wires or metallic traces, and electronic components such as semiconductor chips, transistors, diodes, capacitors, resistors, inductors, etc.;
xe2x80x9celectronic circuit devicexe2x80x9d means a device comprising an electronic circuit or electronic component, such as (1) an electronic package such as a ball grid array (BGA), a laminated microinterconnect (LMI), multi-chip module, or a chip scale package (CSP), (2) simple flexible circuitry wherein copper foil is adhered to a polymer substrate with an adhesive, or (3) an electronic component such as a semiconductor chip connected to a circuit on a substrate;
xe2x80x9cepoxy-bearing materialxe2x80x9d means an epoxy-modified aromatic vinyl-conjugated diene plus epoxy resin, if present;
xe2x80x9csolder resistancexe2x80x9d means resistance to heat at the temperature of solder reflow;
xe2x80x9cresin compositionxe2x80x9d means an epoxy-bearing material plus a curative or catalyst; and
xe2x80x9cepoxy-modified aromatic vinyl-conjugated diene block copolymerxe2x80x9d includes such copolymers that are partially hydrogenated prior to or subsequent to epoxy-modification.
The present invention is advantageous because the copolymer resin composition provides superior solder resistance and moisture insensitivity and uses lamination temperatures (for example, 220xc2x0 C.) much lower than those needed for conventional polyimide-based materials (for example, 350 xc2x0 C.). Furthermore, the copolymer resin composition has excellent peel strength (50 to 325 Newtons/dm) to Kapton(trademark) polyimide film (DuPont), and provides over 1000 hours of performance at 85xc2x0 C./85 R.H. with a 5 volt bias on an interdigitated copper test circuit that has been laminated with the copolymer. The copolymer resin composition has also shown no ink-jet ink sorption by visual observation after immersion for 11 days at room temperature. The copolymer resin composition also has excellent resistance to corrosive, aqueous acidic and/or alkaline environments. Additionally, conventional adhesives and covercoats in electronic packages are known to exhibit moisture-induced failure under solder reflow conditions. The present invention copolymer resin composition does not exhibit moisture-induced failure under solder reflow conditions.